A conventional acoustical stringed instrument comprises a hollow body having a front face or sounding board, a back face which is substantially parallel to the sounding board, and a connecting portion which connects the sounding board to the back face around a perimeter of the respective faces. A longitudinally extending neck member extends from the body and has a distal end having a plurality of string receiving and tighting members. A bridge having a slot therein disposed perpendicularly to the neck member is connected to the sounding board, remote from the neck member. A plurality of strings extend between the bridge and the string receiving and tightening members such that the strings can be releasably placed under tension. A saddle comprising an elongated, narrow strip of hard material, such as ivory, bone or hard plastic, is slideably fitted into the slot in the bridge to support the strings. When the strings are tightened, string tension presses the strings against the saddle and presses the saddle against the bottom of the slot in the bridge. When the instrument is played, vibrational energy from the strings is transmitted through the saddle and the bridge into the sounding board and into the body of the instrument, where it resonates and produces sound.
With the advent of recorded and amplified music, it became necessary to devise a method of electrically capturing the sound from acoustical instruments. This could be accomplished by using microphones placed on or near the instrument, or more conveniently, by placing a "pickup" under the saddle to convert the string vibrations into an electrical signal. An instrument with such a pickup would be an "electro-acoustical" instrument.
A pickup comprises an array of individual piezo-electric crystals which are spaced apart such that each crystal corresponds to one of the strings. Each crystal has one side fixed to a flexible conducting substrate which is connected to a signal ground, and an opposite side connected to an electrical amplifier by a conducting wire. As is well known in the art, a piezo-electric crystal generates a voltage which is proportional to an applied mechanical force. A crystal generates a voltage having a first polarity in response to a compression force and a voltage. having a second, opposite polarity in response to a tension force. Vibrations transmitted from a first string to the saddle result in an applied force at a corresponding first crystal, thereby generating an electrical signal which is proportional to the string vibrations.
As in a conventional acoustic instrument, the saddle is held in the bridge by string tension. The saddle is thereby held in contact with the pickup by string tension, but is not connected to the pickup. Therefore, the saddle can exert a compression force on the pickup but cannot exert a tension force and therefore the pickup cannot generate signals of alternating polarity. However, the string vibrations do alternate between positive and negative maxima and the output from the pickup must be able to accurately follow the string vibrations. One solution has been to apply sufficient string tension to the saddle to cause the saddle to exert a substantially constant biasing force on the pickup which generates a DC biasing voltage in response thereto. When the instrument is played, string vibrations cause differential forces which add to or subtract from the biasing force, resulting in varying compression forces on the pickup and corresponding electrical signals which vary around the DC biasing voltage without changing polarity. The biasing force must be sufficiently large such that the differential forces will not reduce it to zero or the electrical signals will be forced to zero and clipping and distortion will occur.
Difficulties arise in properly loading the pickup such that each crystal has sufficient contact with the saddle and a sufficiently large biasing force applied to it. If the saddle does not contact an individual crystal, no signal will be generated for the corresponding string. If an individual crystal is loaded more or less heavily than the other crystals, the signal volume of the corresponding string will not be balanced with the other strings. These difficulties arise in part because the bridge, in which the saddle and pickup are positioned, is typically made of wood. Wood is heterogeneous, having soft spring wood and hard summer wood, for example. Crystals adjacent soft portions of the bridge may require more biasing force than crystals adjacent hard portions. Matters are further complicated by the fact that the sounding board, upon which the bridge, pickup and saddle rest, may be arched or "bellied-up". The amount of arch typically increases as the strings are tightened such that a saddle which fits correctly when the strings are slackened may not fit when the strings are tightened. The bridge and the piezo-electric pickup are made of relatively flexible materials and will bend to conform with this arched surface but the saddle is made of hard, rigid material and must be shaped to make it conform.
Conventionally, saddles for acoustic instruments have had a continuous lower surface which contacted the bottom of the slot in the bridge so as to efficiently transmit vibrations to the sounding board. However, when using this type of saddle with a pickup, it might be necessary to remove material from the entire length of the lower surface of the saddle to ensure uniform contact with the pick-up when the strings are tightened. Removing material from this relatively large area may result in a laborious shaping operation involving the removal of a great deal of material with files or abrasives. Furthermore, with a large bearing surface, the force from the strings is distributed over the whole length of the saddle and the whole length of the pickup, rather than being concentrated on the respective crystals where it would generate a signal. This results in a loss of efficiency for the pickup and may result in a lower signal to noise ratio.
Calibration of the saddle is achieved by playing each string independently and comparing the relative volumes of the strings. However, calibration of the saddle is rendered more difficult by the continuous lower edge because if the saddle is binding at a point which does not correspond to a crystal, the results of the calibration may be ambiguous. For example, if the bridge does not contact the crystals at the centre of the pickup, it is not readily apparent from which end of the saddle material should be removed. An irregularity at one end of the saddle may be indistinguishable from an irregularity at the other end or both ends of the saddle such that a saddle might be ruined in attempting to achieve the correct shape. If the arch of the sounding board prevents the bridge from contacting the crystals at the ends of the pickup, it is not readily apparent whether to remove material just from the very centre of the saddle or from a larger region centred with respect to the length of the saddle. Removing too much material may silence some of the central strings. While a conservative, iterative process would eventually succeed it shaping the saddle, it would require the repeated tightening and slackening of the strings to insert and remove the saddle and would also require that the strings be re-tightened to the same string tension each time.
There have been attempts to address some of these problems. U.S. Pat. No. 5,644,094 to Dickson discloses a bridge for an acoustic guitar having a backbone portion supported by a plurality of pedestals such that each pedestal supports one string. The pedestals have relatively large bases to more efficiently transfer vibrational energy to a resonant sounding board and are separated from each other by key-hole-shaped spaces which give the pedestals an hour-glass shape. The pedestals are connected to one another by relatively thin connector portions of the backbone portion which permit each pedestal to move relatively independently of the other pedestals so as to permit independent transfer of string vibrational energy. Dickson also discloses the use of two supports, one at each end of the saddle which also contact the sounding board although not directly supporting strings. The Dickson bridge is directed towards use in an acoustic guitar and does not address the requirements of piezo-electric pickups. For example, the enlarged bases, which are desirable for transmitting vibrational energy to a sounding board in an acoustical guitar would result in only a relatively small part of each base actually resting on a crystal in an electro-acoustical guitar, the rest of each base being unnecessarily disposed over the surface of the pick-up such that irregularities in the surface of the instrument or the pickup might interfere with the proper loading of the crystals. The addition of the two end supports would further aggravate this problem. Furthermore, due to the hour-glass shape of the pedestals, if a large amount of material must be removed from their bases, the shape and cross-sectional areas of the pedestals change. This results in a saddle which is unpredictable and difficult to adjust or calibrate.
A saddle for electro-acoustic guitars which is based on the Dickson device is produced by the Fishman company under the trademark "Cleartone Saddle ". The Cleartone saddle is substantially the same as the Dickson device with the exception that the two supports have been combined with two of the pedestals so that the two endmost pedestals are larger than the remaining pedestals. The Cleartone saddle suffers from the same short comings as the Dickson device with the additional problem that the pedestals do not have equal base areas such that unequal pressures may be applied to respective crystals.
Therefore, what is needed is a saddle for a stringed instrument which focuses the string pressure directly onto the piezo-electric crystals of a piezo-electric pickup and which is easily and consistently adjustable so that equal biasing force is applied to each piezo electric crystal.